For many years it has been the practice in the manufacture of ring binders to fabricate the cover for such binders in a three-ply construction. Three rigid or semirigid, rectangular inserts or stiffener panels are heat-sealed between two superimposed sheets of thermoplastic material. Of the three stiffener panels generally used, two of them approximate in size, the back and front panels of the binder. The third panel is a narrower insert strip disposed between the two larger panel to form the back panel or spine of the binder. The inner and outer plastic sheets are fused together or heat-sealed around their peripheral edges. The sheets are also sealed transversely between the adjacent, transverse edges of the cover panel inserts and the back panel inserts. The transverse seals form the hinge areas of the binder. U.S. Pat. No. 3,195,924 is typical of this type of binder construction.
There are several significant disadvantages in a conventional heat-sealed binder cover, particularly along the hinge portions where the cover and the spine or backbone of the binder intersect. Along the hinge lines, the outer plastic sheets are fused or heat-sealed together and their composite thickness is substantially less than the total thickness of the two sheets before the heat-sealing operation. For example: where the two outer sheets of thermoplastic are each 0,015 inch vinyl, the composite thickness, when fused together, is only about 0,020 inch or about 30% less than the total of 0,030 inch where there has been no heat sealing of the plastic sheets. This thickness reduction significantly reduces tensile strength.
In addition, during heat-sealing, the more volatile plasticizers in the vinyl sheet materials are volatilized "off" and the hinge lines tend to be more brittle than the unfused vinyl. This reduces resistance to cracking failure. Moreover, during the heat-sealing operation, the vinyl films along the hinge lines are invariably stretched over the edges of the chipboard inserts when drawn together for sealing so that the plastic sheet material in these areas becomes thinner (thickness reduction to 33% to 50% are typical) than the unsealed vinyl and thus more susceptible to material fatigue failure.
Still another practical problem caused by the shortcomings of the abovedescribed prior art construction might be called spine intrusion. This problem occurs when the binder, filled with pages, is sitting in a vertical position (with the spine vertical) for a long period of time. More specifically, the loose leaf mechanism of the binder is riveted to the spine and loaded with paper. The binder is then placed on a shelf in a vertical position, as normally seen in a book case. The weight of the paper on the top ring rotates the spine inward or forward until the bottom front corner of the edges of the paper comes to rest on the storage shelf. This stress on the spine and hinges causes the spine-edge of the covers to spread outward and the upper part of the spine to move inward between the covers. This happens over a period of time, which varies depending upon the weakness of the hinge, and the weight of the paper. The end result is that usually the upper one third of the spine is bent inward and the outside of the top of the spine is either flush with, or inside the upper edges of the front and back covers of the loose leaf binder. This effect detracts from the appearance of the binder. More importantly, this pulling stress on the hinges frequently causes the hinges to tear, thus damaging the binder. These and other difficulties experienced with the prior an devices have been obviated in a novel manner by the present invention.
The principal object of this invention is to provide an improved ring binder construction and method of fabricating the same which overcome the drawbacks of the prior art construction.
Another object of this invention is to provide an improved ring binder cover having a spine and hinge construction of remarkably enhanced durability without sacrifice in either the appearance or functionality of the binder.
A still further object of this invention is the provision of a plastic-sealed binder which has the appearance of the more expensive case-bound-type binder.
With these and other objects in view, as will be apparent to those skilled in the art, the invention resides in the combination of parts set forth in the specification and covered by the claim appended hereto.